1.1.2　Structural Unit and Repeating Unit

A structural unit（结构单元）is an elementary unit connected to one another in the chain of polymer molecules，or polymeric structure by covalent bonds（共价键）.It is the result of a monomer residue（残留部分） that has been polymerized.All atom groups in each following square frames are called as structural units.

A repeating unit（重复单元）is defined as the shortest sequence that can be found repeatedly in a polymer.This distinction is necessary in order to estimate the molecular weight of polymers correctly.The atom groups in each following square frames are called as repeating units.

The structures of polymers vary largely，but almost all polymers may be expressed as the combinations of many different structure units.In a large number of cases，a single type of structural unit is sufficient for the representation of the entire polymer molecule.This feature is the basic characteristics of polymeric substances（这一特性是高分子物质的基本特征）.

For example，the monomers which could be used to create this polymer polyethylene are ethylene CH₂CH₂.The structural unit and repeating unit both are —CH₂CH₂—.The molecule or structure of polyethylene may be represented by

where n is termed as the degree of polymerization（聚合度）or the number of repeating units in the molecules.　（参见本章二维码：1-1 单体单元和聚合度概念）

Since poly（hexamethylene adipamide）is made from two monomers，HOOC（CH₂）₄COOH and H₂N（CH₂）₆NH₂，there are two structural units in the polymer，which are

—HN（CH₂）₆NH— and —OC（CH₂）₄CO—

The repeating unit is

—HN（CH₂）₆NHOC（CH₂）₄CO—

Therefore，the molecule or structure of poly（hexamethylene adipamide） may be described by

The structural units may be connected together in any imaginable（想得到的）pattern to form so called linear polymer（线型聚合物），branched polymer（支化聚合物），graft polymer（接枝聚合物），block polymer（嵌段聚合物）or crosslinked polymer（交联聚合物）.